A research protocol required that a group of 10 female rhesus monkeys be in the mid-luteal phase of a normal menstrual cycle, all on the same day, so that the same culture of a virus could be used for vaginal inoculation. The monkeys were observed daily for indications of menses. When it occurred each monkey was then injected with progesterone starting on day 16 of the cycle. When all study monkeys had received at least 8 days of progesterone the injections were stopped. The following menses occurred in all monkeys within 2 days of the last progesterone injections. On day 16 of that cycle the monkeys began a 13 day period of progesterone injections. The inoculations were done on a single day during the last series of progesterone. A second study involving 21 female rhesus monkeys is currently underway.